The present invention relates to a method of manufacturing a semiconductor substrate and, more particularly, to a method of manufacturing a semiconductor substrate having a plurality of semiconductor wafers held together by cohesion.
Recently, a technique of bonding semiconductor wafers is preferred for the reduction in a manufacturing cost it achieve. In this bonding technique, at least two semiconductor wafers are polished to have mirror surfaces and directly bonded together, constituting a semiconductor substrate. For example, in the manufacture of a high withstand voltage bipolar transistor of the type having a buried region, the bonding technique allows a step of forming the buried region to be free from a complex conventional process in which diffusion and epitaxial growth would be combined. In this case, a buried region is easily provided by uniting a first semiconductor wafer and a second semiconductor wafer having a diffusion region in its surface area. This type of semiconductor substrates can be mass-produced in advance independently of the manufacture of bipolar transistors with a high withstand voltage, thereby effectively reducing the time required for building high withstand voltage bipolar transistors on one semiconductor substrate.
A method of manufacturing a semiconductor substrate of this type will now be described in detail. To bond two semiconductor wafers, semiconductor wafers are polished at their major surfaces to have mirror surfaces and stacked with the polished surfaces being opposed each other. Thereafter, semiconductor wafers are heated to be held together with sufficient coherency, thus forming a semiconductor body shown in FIG. 1. The semiconductor body is then ground by a predetermined amount to adjust its thickness. Thus, a semiconductor substrate having a thickness suitable for forming semiconductor elements is obtained.
In general, a polishing process cannot maintain a thickness of a semiconductor wafer constant and dulls a peripheral edge of a polished surface as a whole. For this reason, when two semiconductor wafers are polished and stacked together, a wedge-shaped gap is formed between outer peripheries of the semiconductor wafers. This gap remains as a noncohered portion of a semiconductor body after a heating process and causes a part of the semiconductor body to be chipped off during a thickness adjustment process, as shown in FIG. 2. In addition, when a semiconductor substrate is wet-washed prior to the formation of a desired semiconductor element, a wash enters into the gap. The wash scatters from the gap during semiconductor element manufacturing processes, e.g., a heating process or a vacuum deposition and contaminates an essential part of the semiconductor substrate. Accordingly, an outer periphery of the semiconductor body must be ground to be free of the gap described above.